Fiber optic cable having no rigid strength members and a reduced coefficient of thermal expansion

ABSTRACT

A fiber optic cable is provided with an outer jacket material having a coefficient of thermal expansion less than approximately 6E−5 in the range from 23° C. to −50° C. By utilizing an outer jacket material having a reduced coefficient of thermal expansion, the need for rigid strength members within the outer jacket is eliminated, thereby providing a fiber optic cable having a reduced size and weight.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates generally to a fiber optic cable. Moreparticularly, the present invention relates to a fiber optic cablehaving no rigid strength members, a reduced weight, and a reducedcoefficient of thermal expansion.

2. Related Art

A conventional central tube fiber optic cable design includes rigidstrength members embedded in an outer jacket of the cable. The rigidstrength members are utilized for tensile strength and to preventattenuation in the optical fibers within the cable due to thermalcontraction of the cable.

While the rigid strength members are effective in reducing the thermalcontraction of the cable, the use of rigid strength members introducesnumerous drawbacks. For example, the presence of rigid strength membersin the outer jacket significantly increases the wall thickness of theouter jacket, the overall diameter of the fiber optic cable, and theweight of the fiber optic cable.

Another drawback associated with the use of rigid strength members isthat the bending performance of the fiber optic cable is significantlylimited. In the traditional fiber optic cable, two rigid strengthmembers are generally positioned so as to be diametrically opposed toone another within the outer jacket of the cable. Due to the presence ofthese rigid strength members, the fiber optical cable exhibits apreferential and a non-preferential bending direction. Such bendingrestrictions negatively affect the ease of installation of conventionalfiber optic cables.

Therefore, what is needed is a fiber optic cable design that eliminatesthe need for rigid strength members while preventing the thermalcontraction of the outer jacket material.

SUMMARY OF THE INVENTION

A fiber optic cable is provided with an outer jacket material having acoefficient of thermal expansion less than approximately 6E−5 in therange from 23° C. to −50° C. By utilizing an outer jacket materialhaving a reduced coefficient of thermal expansion, the need for rigidstrength members within the outer jacket is eliminated, therebyproviding a fiber optic cable having a reduced size and weight.

The above and other features of the invention including various andnovel details of construction and combination of parts will now be morefully described with reference to the accompanying drawings and pointedout in the claims. It will be understood that the particular featuresembodying the invention are shown by way of illustration only and not asa limitation of the invention. The principles and features of thisinvention may be employed in varied and numerous embodiments withoutdeparting from the scope of the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

Aspects of illustrative, non-limiting embodiments of the presentinvention will become more apparent by describing in detail embodimentsthereof with reference to the attached drawings in which:

FIG. 1 is a cross-sectional view depicting a conventional fiber opticcable.

FIG. 2 is a cross-sectional view depicting a fiber optic cable accordingto an illustrative embodiment of the present invention.

FIG. 3 is a graph depicting the coefficient of thermal expansion atdifferent temperatures for various outer jacketing materials.

DETAILED DESCRIPTION OF THE INVENTION

The following description of illustrative non-limiting embodiments ofthe invention discloses specific configurations, features, andoperations. However, the embodiments are merely examples of the presentinvention, and thus, the specific features described below are merelyused to more easily describe such embodiments and to provide an overallunderstanding of the present invention.

Accordingly, one skilled in the art will readily recognize that thepresent invention is not limited to the specific embodiments describedbelow. Furthermore, the description of various configurations, features,and operations of the present invention that are known to one skilled inthe art are omitted for the sake of clarity and brevity. Also, it is tobe understood that the phraseology and terminology employed herein isfor the purpose of description and should not be regarded as limiting.

FIG. 1 shows a cross-sectional view of a conventional fiber optic cable.The cable is provided with a core having swellable yarns 102 and aplurality of buffer tubes 104. Each buffer tube 104 holds a plurality ofoptical fibers 106 and is filled with a filling compound 108. Buffertubes 104 are surrounded by a water swellable tape 110 which preventsthe ingress of water.

An inner tube 112 surrounds water swellable tape 110 and is providedwith a rip thread 114 to allow the removal of overlying layers. Strengthyarns 116 surround inner tube 112 and are provided with a ripcord 118.Surrounding strength yarns 116 is an armor layer 120 which providesadded support for the cable.

Finally, an outer jacket 122 is provided having a plurality of rigidstrength members 124. To accommodate rigid strength members 124, outerjacket 122 must be formed with a thickness of approximately 2.25 mm.Conventional materials utilized for outer jacket 122 include highdensity polyethylene (HDPE), medium density polyethylene (MDPE) andlinear low density polyethylene (LLDPE).

These conventional materials have a substantially higher coefficient ofthermal expansion than that of glass. Therefore, differential thermalstrains are developed as a function of temperature variation. At lowtemperatures, the conventional outer jacketing materials contractsubstantially more than the glass fibers in the cable, therebypresenting a possible risk of fiber strain due to contraction and/orfiber buckling. Such strain and buckling induced on an optical fiberwill result in attenuation and possible network system failure. Therigid strength members 124 are included within outer jacket 122 to limitsuch cable contraction at low temperatures.

FIG. 2 shows a fiber optic cable according to an illustrative embodimentof the present invention. The cable is provided with a core havingswellable yarns 202 and a plurality of buffer tubes 204. Each buffertube 204 holds a plurality of optical fibers 206 and is filled with afilling compound 208. Buffer tubes 204 are surrounded by a waterswellable tape 210 which prevents the ingress of water.

An inner tube 212 surrounds water swellable tape 210 and strength yarns214 which provide tensile strength for the cable surround inner tube212. Ripcord 216 is provided which allows the removal of any overlyinglayers. Surrounding strength yarns 214 is an armor layer 218 whichprovides added support for the cable.

Finally, the cable is provided with an outer jacket 220. Outer jacket220 is made from a material having a coefficient of thermal expansionless than approximately 6E−5 in the range from 23° C. to −50° C. Anexample of a material for outer jacket 220 includes, but is not limitedto, a nucleated impact polypropylene having a coefficient of thermalexpansion less than approximately 6E−5 in the range from 23° C. to −50°C.

By utilizing such a material, the need for rigid strength members 124 iseliminated. Instead, the outer jacket itself is able to protect theoptical fibers from strain and/or buckling at low temperatures, therebypreventing attenuation and potential network system failure. In anillustrative embodiment of the present invention, the thickness of theouter jacket is approximately 1.00 mm, significantly less than thethickness required by the conventional outer jacket.

FIG. 3 is a graph depicting the coefficient of thermal expansion over arange of temperatures for nucleated impact polypropylene, high densitypolyethylene (HDPE), medium density polyethylene (MDPE) and linear lowdensity polyethylene (LLDPE). As is clearly shown, nucleated impactpolypropylene has an almost 90% lower coefficient of thermal expansionat −25° C. and a 60% lower coefficient of thermal expansion at 0° C.than the various polyethylene materials.

By utilizing an outer jacket material having a reduced coefficient ofthermal expansion, and thereby eliminating the need for rigid strengthmembers within the outer jacket, the fiber optic cable of the presentinvention is able to achieve a reduced weight, a reduced diameter, nopreferential bending direction, improved installation distance duringblowing or other installation, increased ease of routing in tight accessnetwork environments and reduced material usage and cost.

The present invention is not limited for use with a central tube fiberoptic cable having buffer tubes. For example, the present invention maybe utilized in conjunction with a loose tube cable in which tubescontaining fibers are stranded around a central strength member. Inaddition, the present invention may be utilized in conjunction withcables having fibers or fiber bundles which float freely in the centertube, or cables having a ribbon stack in the center of the cable.

The previous description of embodiments is provided to enable a personskilled in the art to make and use the present invention. Moreover,various modifications to these embodiments will be readily apparent tothose skilled in the art, and the generic principles and specificexamples defined herein may be applied to other embodiments without theuse of inventive faculty. Therefore, the present invention is notintended to be limited to the embodiments described herein but is to beaccorded the widest scope as defined by the limitations of the claimsand equivalents.

1. A cable comprising: an outerjacket material forming an outermostlayer of the cable, said outer jacket material having a coefficient ofthermal expansion less than approximately 6E−5 in the range from 23° C.to −50° C.; at least one optical fiber surrounded by said outer jacketmaterial; and a buffer tube containing said at least one optical fiber,wherein said outer jacket material comprises a nucleated impactpolypropylene, and wherein said outer jacket material does not containrigid strength members.
 2. A cable according to claim 1, wherein saidouter jacket material does not contain rigid strength members.
 3. Acable according to claim 1, wherein the cable does not contain rigidstrength members.
 4. A cable according to claim 1, further comprising:at least one optical fiber, wherein said at least one optical fiber issurrounded by said outer jacket material, and wherein said outer jacketmaterial forms an outermost layer of the cable.
 5. A cable, comprising:an outer jacket material forming an outermost layer of the cable, saidouter jacket material having a coefficient of thermal expansion lessthen approximately 6E−5 in the range from 23° C. to −50° C.; at leastone optical fiber surrounded by said outer jacket material; and a buffertube containing said at least one optical fiber, wherein said outerjacket material comprises a nucleated impact polypropylene and whereinthe cable does not contain rigid strength members.